Rework X selections to make it safe to run the debugger inside converters

Rework X selections to make it safe to run the debugger inside converters

* src/keyboard.c (prev_kbd_event): Delete function.
(readable_events): Return 1 if
x_detect_pending_selection_requests returns true.
(kbd_buffer_unget_event): Also delete function, since nested
selection requests are really handled correctly.
(kbd_buffer_get_event): Handle events from the special X
deferred selection queue as well.

* src/keyboard.h: Update prototypes.

* src/xselect.c (struct selection_event_queue)
(selection_input_event_equal, x_queue_event)
(x_start_queuing_selection_requests)
(x_stop_queuing_selection_requests): Delete structs, since they
are no longer required.
(x_handle_selection_request, x_handle_selection_event): Allow
nested selection events.

* src/xterm.c (struct x_selection_request_event): New struct.
(x_handle_pending_selection_requests_1)
(x_handle_pending_selection_requests): Handle all events in the
new selection event queue.
(x_push_selection_request, x_detect_pending_selection_requests):
New functions.
(x_dnd_begin_drag_and_drop): Drain the selection queue here as
well.
(handle_one_xevent): When inside a nested event loop, just push
selections to that queue.
(XTread_socket): Allow reading X events if x_dnd_unwind_flag is
true, even though DND is in progress.
(x_delete_display): Delete pending selection events for the
display that is going away.

* src/xterm.h: Update prototypes.